sh: Use exist program

Fix sh module packages to make more extensive use of the exist program
that also ships with the sh module.
Simplifies intent of the code and makes it much easier to read.

sh/.config/sh/alias

@@ -61,6 +61,8 @@ if exist fzf; then
         fzf_pkg_tool=yay
     elif exist paru; then
         fzf_pkg_tool=paru
+    elif exist pacman; then
+        fzf_pkg_tool=pacman
     fi
     # shellcheck disable=2139 # we *want* this to be done at shell startup instead of dynamically
     if [ -n "$fzf_pkg_tool" ]; then
@@ -89,6 +91,4 @@ fi
 if exist vifm; then
     alias vm=vifm
     alias vmm='vifm ${PWD}'
-    # enable picture preview script
-    exist vifmrun && alias vifm=vifmrun
 fi